We propose to study in vitro a human suppressor monocyte/macrophage system. Our data which provide the background for this proposal show that suppressor monocyte/macrophages can be elicited by in vitro culturing of normal, peripheral blood mononuclear cells. These suppressor cells can be purified by Ficoll density gradient centrifugation. The cultured monocyte/macrophages are capable of suppressing allogeneic stimulation, generation of cytotoxic cells and mitogenic stimulation. We are able to extract a soluble factor from such cells which itself can suppress the mixed lymphocyte reaction. Using a panel of fully HLA-A, -B, -C and -DR typed normal donors, our objective is to: a) study the importance of HLA-DR (Ia) specificities at the level of suppressor monocyte/macrophages and at the level of suppressor soluble factors as each interacts with HLA-DR typed responder cells stimulated by alloantigen or by pokeweed mitogen (PWM). We shall measure alloantigen stimulation by 3H-thymidine incorporation and mitogenic stimulation by immunoglobulin production, using a radioimmunoassay. The suppressor values obtained in checkerboard-type cell combinations will be used to a) evaluate clustering of suppression values in relation to the number of HLA-DR specificities shared, i.e., two, one or none between each suppressor and responder cell pair; b) characterize the soluble factor from physical and immunochemical points of view; this will include: stability, molecular weight, solid phase immunoadsorptions by the use of Sepharose coupled xenoantisera directed against: human immunoglobulin, human beta 2 microglobulin, human p29,34 (Ia); c) establish the antigenic uniqueness of suppressor monocyte/macrophages from the point of view of cell surface differentiation antigens by preparing xenoantisera to these cells and by rendering them specific through successive adsorptions with Sepharose-pooled human serum, autologous red blood cells, autologous T cells, Daudi cells and eventual adsorption and elution from chemically fixed autologous suppressor monocyte/macrophage cells.